NO BRAKES/ 1991 ford f150

My old F150 truck for some reason lost all its brake pedal suddenly. The brakes were all woking fine but then today the pedal went to the floor without any warning. It was ok when parked and then no brakes a few hours later.

What would be the likely cause to this. Could it be the brake booster of master cylinder?

Booster???.........Nope.......A booster failure leaves you with a high rock hard pedal that DOES NOT stop the vehicle........
Master Cyl ???........maybe.........Process of elimination
You need to check ALL 4 wheels for fluid leaks AND all undervehicle lines ........metal and rubber. If everything is A#1 at that point........you need a master cyl (also confirm there is fluid in the reservoir)

Wrench thanks for the input. The truck has been driven by my son the last few weeks with no problems, but the a/c quit so he dropped it off at the a/c shop, and they said it had no brakes when they moved the car into their shop.

First thought was they were wanting to do a brake job too but then they informed me that they don't do anything but a/c work and wouldn't touch the brakes. I have also dealt with this shop before and found them to be very honest.

When they get the a/c up and running I will have to deal with the brakes but I almost think the person who drove it into the shop is wrong. Doesn't a master cyl have two separate systems so you don't loose both front and back brakes at the same time. They told me the brakes went to the floor!

Older Fords are notorious for master cylinder failure. You are right, it does have two reservoirs so the whole system is not lost when a line fails but a leaking piston will affect both of them. Was the fluid so low that air was introduced into the system? Will the brakes work if the pedal is pumped?